Accumulation of alpha-synuclein (?Syn) causes degeneration of dopaminergic (DA) and non-DA neurons in Parkinson?s disease (PD) and Dementia with Lewy Bodies (DLB). ?Syn also contributes to the fibrilization of amyloid-? and tau, two key proteins in Alzheimer?s disease (AD), which suggests a key role for ?Syn toxicity in neurodegeneration. Thus, it is important to elucidate downstream effects and the factors promoting the toxic conversion of ?Syn, towards understanding the pathogenesis of and developing disease-modifying therapies for synucleinopathies. In PD and DLB, pathological ?Syn proteins enrich in mitochondria of vulnerable brain regions, where to induce mitochondrial bioenergetic defects and production of reactive oxygen species. Despite evidence suggests that ?Syn-mitochondrial interactions may play a causal role in PD and DLB, the field lacks a detailed understanding of the mechanisms by which ?Syn abnormality and mitochondrial functional deficiency influence each other. To maintain normal mitochondrial health and function, cells employ a mitochondria-to-nucleus signaling pathway termed the mitochondrial unfolded protein response (UPRmt). The UPRmt monitors mitochondrial proteostasis through mitochondrial specific proteases and molecular chaperones, which facilitate folding and/or degradation of unfolded proteins within mitochondria. They also communicate with the nucleus by retrograde signaling to activate the expression of peptide-folding related proteins. The UPRmt is an important defense mechanism for maintaining the quality of proteins within the mitochondria under stress. Defects in UPRmt have been linked to aging and neurodegeneration. Our recent work showed, for the first time, that the protein level of ClpP, a mitochondrial matrix protease induced during UPRmt activation, was decreased in neurons expressing ?Syn wildtype (WT) or A53T mutant, in brains of mice carrying A53T mutant, and in the Substantia Nigra of PD patients. The mRNA level of ClpP remained unchanged, suggesting a transcriptional independent effect. Preliminary study further found that the protein level of ClpP and not other mitochondrial proteases, decreased in the cortex of patients with DLB and mice expressing human Thy1-?Syn. These results suggest that a decrease in ClpP is a common event implicated in the pathogenesis of both PD and DLB. Whereas silencing ClpP in neurons increased a load of unfolded proteins in the mitochondria, reduced mitochondrial bioenergetic activity and increased cell death; overexpressing ClpP abolished ?Syn-induced oxidative stress in cultured cells, and attenuated ?Syn hyper-phosphorylation and behavioral abnormality in ?Syn A53T mice. Notably, we found that ?Syn bound to ClpP and suppressed ClpP peptidase activity, whereas genetic manipulation of ClpP influenced the assembly of non-toxic ?Syn tetramers that resist aggregation. Thus, our pilot findings highlight a previously unidentified interdependence between pathological ?Syn and mitochondrial protease ClpP, which results in a disturbance of mitochondrial proteostasis, leading to neuronal damage. Given that ?Syn accumulation is a common pathological hallmark of both PD and DLB, the goal of this study is to determine the role of ClpP-mediated mitochondrial proteostasis in PD and DLB at both mechanistic and therapeutic level. Built on our study supported by the NIH bridge award R56 NS105632-1A1, we will test the central hypothesis that pathological ?Syn disturbs mitochondrial proteostasis by suppressing ClpP and UPRmt, which impairs mitochondrial bioenergetic activity and promotes the toxic conversion of ?Syn, leading to ?Syn neuropathology. Our research team has the unique synergistic expertise in UPRmt, mitochondrial biology, and ?Syn neuropathology required to impact this significant area of unmet medical need. Successful completion of the proposed study will not only contribute to the basic understanding of disease pathogenesis, but it will also aid in the development of treatments for PD and DLB and even other neurodegenerative diseases in which ?Syn aggregation manifests.